It seems hard to believe that any movie could show off a different side of Christian Slater at this point, more than 25 years after he became a sensation thanks to "Heathers."

But working with famed enfant terrible Lars von Trier ("Melancholia") on a film called "Nymphomaniac" will have that effect.

So it is that Slater plays the father of sex-addicted protagonist Joe, rhapsodizing about the trees and enduring a painful, prolonged hospital scene.

Slater, 44, had some nerves about taking on such a difficult role in the film, the first part of which is on video on demand and in theaters Friday.

"That first day of school, or that first day of camp, is always very nerve-racking and intimidating," Slater says. "You don't know how it's going to go or who's going to be your buddy or if you're going to be bullied."

But von Trier, contrary to his reputation of being a taskmaster, set him at ease. "I found him to be so surprisingly sweet and genuine and down to earth that I immediately felt very safe," Slater says of their first meeting.

Still, "I was very scared how we were going to capture some of those hospital scenes," Slater says.

It's not easy to be this vulnerable and emotional on screen, even for a veteran performer.

NYC, in your inbox.

"Actors are very, very sensitive creatures," he says. "We really put ourselves out there. ... We're lightning rods of sensitivity, we feel things very very deeply, and when somebody hurts us, it really, really hurts. We are also very hard on ourselves, so mentally it's very challenging. You put yourself out there and you hope for the best."